The 52-year-old actor, who starred in the 1997 film, My Best Friend's Wedding, with Roberts, couldn't stop gushing over the brunette beauty when ET caught up with him on the red carpet at the Spike TV Guys' Choice Awards in Culver City, California, on Saturday.

Mulroney told ET's Katie Krause that he was ecstatic to be presenting the Woman of the Decade award to his former co-star, whom he still considers a close friend.

"I'd walk a mile on broken glass for her, so this was a lot easier to come in to Culver City," he joked.

"It doesn't always happen with every person that you love and work with -- you kind of go your separate ways," he added. "But we, against all odds, managed to maintain a great close friendship. So, I'm happy to be here tonight."

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Mulroney further explained that it's Roberts' genuine character and funny personality that's kept them tight-knit after all these years.

"She's just one of the most genuine friends I have," he shared. "She's a really generous person -- great sense of humor and really caring. Great family, and she's been a good friend and role model for me."

Genuine, indeed! Roberts, 48, was all smiles as she accepted her award on the casino-themed stage, turning heads in an off-the-shoulder black Yigal Azrouel jumpsuit with lace detailing.

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Aside from his friendship with Roberts, Mulroney also got candid about his role on the hit Showtime series, Shameless.

On the season six finale -- SPOILER ALERT! -- Mulroney's character, Sean, was set to wed Fiona, portrayed by Emmy Rossum, in a ceremony from hell. In the final scene from the episode, fans saw a teary-eyed Fiona dressed in a wedding dress and smoking a cigarette alone on a curb. Sean eventually meets her outside, seemingly saying goodbye to her forever.

"I don't know how that's going to work. It's a mystery," Mulroney said about what's to come of his character when the series returns in October. "I can tell you that with Shameless, we never knew [what] the next week's plot was going to hold. It's half of the fun of being on a show like that, because you know it's always going to be good, but it always takes a left turn and surprises you with the storyline. I know they'll come up with something great."

He also told ET that he's had a lot of fun playing Sean, specifically because so many viewers can relate to his beloved character.

"I think a lot of people have shared with me that they know guys like that, or people in their families," he explained. "So there's something about that character that I think really touched people. To be included into Shameless -- an up-and-running [show] that's so popular -- is a great privilege for me. But I felt some responsibility for that character, too."

As we patiently wait for Shameless to return, and for the Guys' Choice Awards to air June 9 on Spike TV, watch the video below to see more sneak peeks from the star-studded awards show, including a close-up look at Chrissy Teigen's very sexy ensemble.